MEMS (microelectromechanical systems) resonators are small electromechanical structures that vibrate at precise frequencies. MEMS resonators are useful in electronic circuits for providing timing references and frequency references. In typical applications, a MEMS resonator is attached to an electronic circuit to form an oscillator circuit. A MEMS oscillator includes a MEMS resonator driven by a sustaining amplifier in continuous motion. The mechanical resonant vibration of the MEMS resonator is sensed and converted into an electrical signal with a very precise frequency. The precise MEMS resonant frequency is used as the reference frequency for the oscillator circuit. The electronic circuit attached to the MEMS resonator amplifies the sensed electrical signal and sets or adjusts the output frequency of the oscillator based on the MEMS resonant frequency. For example, the electronic circuit may include a phase-locked loop (PLL) or a frequency-locked loop that generates programmable output frequencies based on the MEMS resonant frequency as the reference frequency.
Common applications for MEMS oscillators include real-time clocks. A real-time clock (RTC) is a computer clock, often in the form of an integrated circuit, used to keep track of the current time in electronic systems, such as computers, servers and consumer electronic devices. FIG. 1, which includes FIGS. 1(a) and 1(b), illustrates conventional real-time clock circuits. Referring to FIG. 1(a), a real-time clock 1, often provided as a real-time clock integrated circuit, includes an oscillator circuit 2 and supporting circuitry (RTC circuit) 3. Traditional real-time clocks use a crystal oscillator circuit that uses the mechanical resonance of a vibrating quartz crystal 4 to provide the desired reference frequency. The quartz crystal 4, a discrete component outside of the real-time clock integrated circuit, is driven to resonate at a desired frequency, such as 32.768 kHz. The oscillator circuit 2 turns the vibration of the quartz crystal 4 into an electrical signal with the desired precise frequency (e.g. 32.768 kHz). The RTC circuit 3 provides signal amplification, clock division, and other time keeping functions. The real-time clock 1 often includes an alternate power source 5 so that the real-time clock can continue to keep time while the primary source of power is off or becomes unavailable. The alternate power source 5 can be a battery power source, such as a lithium ion battery or a supercapacitor.
Because quartz crystal is bulky and does not integrate well with semiconductor integrated circuits, MEMS resonators have become an attractive alternative to the traditional quartz crystal in constructing oscillator circuits. Referring to FIG. 1(b), a real-time clock 6 is formed using a real-time clock integrated circuit that includes a real-time clock chip 7 and a mems resonator 8, which can be co-packaged within the same integrated circuit package, such as, but not limited to, a quad flat no-leads package (QFN) or a land grid array package (LGA). The MEMS resonator 8 provides a precise reference frequency. The real-time clock chip 7 houses all of the supporting circuitry, including the oscillator circuit 2 and the RTC circuit 3. With the MEMS resonator thus integrated, the size of the real-time clock is reduced. Furthermore, MEMS resonator provide additional benefits such as more consistent stability over a wider temperature range and better resistance to environmental factors such as shock and vibration.